The velocity increase isn't surprising, generally speaking longer rifle barrels like to yield higher velocities.
Pierced primers is unlikely an effect of your longer barrel.
The pressure peaks within inches of the bullet beginning to move, not 30" away.
You would look at other causes of excess pressure.
Make sure you are measuring the powder correctly, check your
chamber dimensions, weigh your brass to see if it happens to be heavy
(heavier weight means less capacity within the case), weight your bullets
to make sure they are the weight you think, compare bullet and groove diameter,
etc. etc. etc.
If you cannot find any cause of the increased pressure, you can compensate for it.
Since you get extra velocity with the extended barrel, you can afford to
cut down on powder charge to reduce pressure (and barrel erosion).
Although it may be counter-intuitive, you can also shorten your OAL.
Like Weatherby cartridges, you give the bullet a longer jump to the lands
and it helps smooth out maximum pressure at the chamber.
I know common lore says it is more accurate to get the bullet closer to the lands,
but when your pressure is blowing primers you need to address pressure first.
And many rifles (like Weatherby) do not suffer from the jump. They are accurate.